Data link layer elementry protocols
- 1. Data Link Layer Protocols Done by Mrs.R.Sabitha
- 2. Flow Control: Initial Assumptions • Simplex Channel • Infinite buffer capacity with the receiver • Error free transmission • Network layer at the senders end is always ready with data • No need for flow control
- 6. • Infinite capacity in the buffer of the receiver. • Need for “flow control” • Stop-n-Wait protocol – Sender sends a frame – And waits for a signal in the form of a dummy frame – No seq no. is required since the line is still error free
- 8. • The channel is noisy, frames may be damaged or lost • Advantage : data frame reaches intact, ack sent back and received, next frame sent • Disadvantage : – Data frame damaged or lost ..hence no ack – sender times out and resends .. No problems – Data frame reaches intact but Ack lost .. Times out ..resends.. Receiver receives duplicate frames..Problem
- 9. Problem of Duplicate frames • Solution : – Keep a sequence number for each frame to distinguish between the new frame and a duplicate frame. • How large should be the sequence number? Or • What should be minimum number of bits required for the sequence number? – The only ambiguity at the receiver is between two successive frames.. Say m and m+1 and never between m-1 and m +1 .. Its only after the ack for m-1 reaches back intact mth frame is sent.. And once mth frame reaches intact at the receiver’s end .. Story of m-1th frame is over .. However depending upon whether ack of mth frame reaches back intact or not either mth or m+1 th frame is sent .. hence only 1 bit is sufficient
- 10. A Simplex Protocol for a Noisy Channel A positive acknowledgement with retransmission protocol. Continued
- 11. A Simplex Protocol for a Noisy Channel (ctd.) A positive acknowledgement with retransmission protocol.
- 12. • Channel is simplex .. • So, Now communication is two way .. I.e. Duplex channel – Interleave data and ack .. So ..a kind field is required to specify whether the frame contains data or ack. – Piggyback ack on data frame
- 13. Sliding Window Protocol(Duplex) • At any instant of time, the sender maintains a set of sequence numbers corresponding to frames it is permitted to send – this includes – frames not yet sent – Frames sent but not yet acknowledged .. May have to be sent again This corresponds to the sender’s window. When a packet comes from the NL ..it is appended in the window and when an ack arrives for a frame..it is deleted from the buffer(and from the window)
- 14. Sliding Window Protocol(Duplex) • Similarly, the receiver also maintains a set of sequence numbers corresponding to frames it is permitted to accept. – When a frame arrives, its seq number is checked..if it falls in the window it is accepted else discarded .. Once an ack is sent for a frame it is deleted from the window so that duplicate frames are not accepted. • Sliding window gives the DLL the flexibility to accept the frames in any order but they must be handed over to the NL in order.
- 15. Sliding Window Protocols (2) A sliding window of size 1, with a 3-bit sequence number. (a) Initially. (b) After the first frame has been sent. (c) After the first frame has been received. (d) After the first acknowledgement has been received.
- 16. Sliding Window Protocols • A One-Bit Sliding Window Protocol • A Protocol Using Go Back N • A Protocol Using Selective Repeat
- 17. A One-Bit Sliding Window Protocol Continued